Respiratory illness not a precursor of pertussis

New research has found that contrary to some recent studies, only a low percentage of children treated for respiratory illness with cough have unsuspected pertussis.

Although some children with respiratory viral illnesses are also infected with the Bordetella bacteria that cause pertussis, the most likely explanation is coincidence rather than any predisposition to secondary infection, according to the new study by Swiss researchers.

The researchers analyzed nasopharyngeal swabs from 1,059 infants, children and adolescents with cough illness treated over a one-year period. In addition to respiratory viruses, the swabs were tested for Bordetella bacteria, including Bordetella pertussis — the main cause of pertussis. They were also tested for a related species, Bordetella parapertussis, which can also cause pertussis.

In addition, the researchers looked at how often children with common respiratory viruses had concurrent infection with Bordetella bacteria. According to the researchers, some recent studies have suggested that many children with cough related to common viral infections may also have unsuspected Bordetella infection.

The tests showed low rates of Bordetella infection: 2% for B. pertussis and 0.5% for B. parapertussis. The rates were low even for children whose doctor ordered a test for Bordetella to check for possible pertussis.

Rates of Bordetella infection were also low in children with confirmed respiratory viral infections. Of 268 patients who tested positive for respiratory syncytial virus — a common respiratory infection in infants — only one was also infected with Bordetella.

Although routine vaccination is effective, outbreaks of pertussis still occur. The researchers said the new findings can reassure doctors that they are not overlooking unsuspected cases of pertussis in infants and children with cough illness, the vast majority of which are caused by RSV or other viruses.

The researchers said the study also provides no evidence that infection with respiratory viruses makes children more vulnerable to Bordetella infection. When the infections occur together, it's most likely a coincidence, rather than any special susceptibility, the researchers said.

At the same time, they emphasized that detecting RSV or other respiratory viruses does not rule out concurrent infection with Bordetella. They also acknowledged some important limitations of their study, including the fact that it was carried out in a single year during which there were no pertussis outbreaks.

The study appears in the August issue of the Pediatric Infectious Disease Journal: http://bit.ly/npEjLn.